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VI. The Prospects of Improvements in Other Plants

VI. The Prospects of Improvements in Other Plants

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yet known whether the accession grown in another year would again

have a high lysine content.

Assuming that one could detect mutations that suppress prolamine

synthesis and significantly change the prolamine: glutelin ratio in any of

the cereals rich in alcohol-soluble proteins, the results might not be as

striking as with the o1 and j12 mutations in maize as Mertz (1969) has

pointed out. The prolamines of cereals can be divided into 3 groups on

the basis of their amino acid composition: the first group contains the

gliadin of wheat, hordein of barley, and secalin of rye; the second contains the zein of maize, panicin of millet; the third contains the avenin of

oats (Mosse, 1968). The prolamines of the second group contain very low

amounts of lysine. To this group can be added kafirin of sorghum with a

lysine content of less than 0.2 g. per I00 g. of protein. The consequence of

suppressing prolamine synthesis with consequent compensatory synthesis of other protein fractions would be expected to be greater in terms

of lysine content for maize, millet, and sorghum than for cereals of the

first group, where the lysine content of the prolamine fraction is ca. 1 g.

per 100 g. of protein.

The possibility of raising the methionine content in the proteins of the

legumes deserves careful study since methionine is the limiting amino

acid for all legumes. For obvious reasons, the proteins of the soybean

seed have been intensively investigated. As in the other leguminous

seeds, the bulk of the seed protein is globulin in nature and was once

thought to be a homogeneous protein (glycinin). Ultracentrifugal studies

have revealed that 4 components (2, 7, 1 1 , and 15 S) are present (Naismith, 1955). Wolf and Sly ( 1 967) have shown that other methods of

fractionation will separate the components. Roberts and Briggs ( 1 965)

reported that the 7 S component that comprises 30 percent of the total

protein has an extremely low methionine content-0.19 g. per 100 g. of

protein. For comparison, the entire globulin fraction has a methionine

content of 1.4 g. per 100 g. of protein. The 7 S fraction also differs appreciably from the total in its content of threonine and glycine. If the

synthesis of the 7 S fraction could be blocked or suppressed genetically,

and compensatory synthesis of the other fractions resulted, the methionine content would be raised substantially. Wolf et al. (1961) have reported that the relative proportions of the 7 S and 1 1 S fractions were

quite different in Clark soybeans grown in Illinois and Hakuhou No. 1

soybeans grown in Japan. In this instance, it is not clear whether variety,

location, or both are responsible. The possibility that lines with a low

quantity of the 7 S fraction exist, or could be induced, should be investigated.

In this review, the principal concern has been the enhancement of



biological value of cereal and legume seed proteins. It appears that this

is most likely to be achieved by altering the relative proportions of

storage proteins that have different amino acid compositions, but changes

in nutritional value may arise through other circumstances. In the cereals,

the proteins of the germ are much superior to those of the endosperm in

nutritive value. Tables I 1 and I11 demonstrate this point for maize. If a

greater proportion of the protein were germ protein, the biological value

of the protein would be increased. I t is possible that the differences in

lysine content in different races of Mexican maize (Tell0 et al., 1965) may

be explained by varying germ :endosperm ratios.

Potato tubers from different varieties may have a 2-fold range in the

content of such essential amino acids as lysine and methionine (Nehring

and Schwerdtfeger, 1957). Reissig ( 1958) has shown that potato tubers

have a substantial portion of their total nitrogen as nonprotein nitrogen

(free amino acids). The true protein fraction as distinguished from crude

protein (the total nitrogen content x 6.25) has a good amino acid balance.

The proportion of nonprotein nitrogen varies in different varieties (40-54

percent). The content of essential amino acids expressed as an EAA

index (Oser, 195 1 ) was much higher in the protein fraction (EAA index

83 to 89 percent) than in the nonprotein fraction (EAA index 31 to 43

percent). The protein content was highly correlated with the length of the

growing season- the later the variety, the greater the percentage of

nitrogen that was present in the protein fraction and the higher the EAA

index. Within maturity groups, there were still differences between

varieties as to the percentage of nitrogen present as true protein. The biological value of potatoes could be raised by selection for lines that could

synthesize larger quantities of protein within a given maturity season.

Toxic substances present in seeds can be important deterrents to the

use of their proteins. Liener (1966) has reviewed the subject of both

proteinaceous and nonproteinaceous substances in seeds that present

problems because of their toxicity. The legumes as a group contain an

array of antinutritional factors - trypsin inhibitors, hemagglutinins, and

goitrogenic substances. Since these compounds can be destroyed by the

proper heat treatments, no program to lower their concentration in the

seeds seems justified. An exception is Lathyrus sativus, cultivated on 5

million acres in India. Consumption of this legume can result in permanent paralysis apparently caused by P-N-oxalyl-a,P-diaminopropionic

acid. Although the toxic substance can be extracted by thorough cooking,

the cooking water being discarded, or by soaking in cold water and steeping in hot water (Mohan er al., 1966), it would obviously be desirable to

identify strains lacking the toxin or having a very low content.

The use of meals remaining after oil is extracted from the seeds of a



number of cruciferous plants is limited by the presence of thioglycosides

that are enzymatically hydrolyzed to yield goitrogenic isothiocyanates.

As the hydrolytic enzymes are present in the meal, the meal can be

moistened to enable hydrolysis to occur. The isothiocyanates can then

be removed by steam distillation. Lines of Brassica carrzpestris much

lower in thioglycosides have been identified (Josefsson and Appelqvist,

1969). Possibly such strains can be used in breeding programs to achieve

varieties sufficiently low in thioglycosides to be used without hydrolysis

and distillation.

In order to utilize the proteins of cottonseed meal in animal or human

diets, it is necessary to remove the toxic pigment gossypol by solvent

extraction. Strains of cotton low in gossypol can be selected (Rhyne

et al., 1959). Meal from these strains is equal in nutritive value to that of

solvent-extracted commercial meal (F. H. Smith et al., 1961). Gossypol

is eliminated altogether in genotypes lacking the pigment glands in which

gossypol is produced (McMichael, 1960). If mutant plants produce fibers

equal to normal plants in quality, the introduction of the mutation into all

commercial varieties would make the use of unextracted cottonseed meal


One other possibility deserves a brief mention. The cereal grains contain only low quantities of free amino acids. The production of any amino

acid is evidently regulated to correspond closely to the demand for it in

protein synthesis and other reactions. The mechanism(s) of such regulation has not been intensively investigated in higher plants, but much is

known about the regulation of amino acid synthesis in microorganisms

where one or more enzymatically mediated reactions may be key reactions from a regulatory standpoint. A mutation may cause the loss of

sensitivity to the usual signals repressing enzyme synthesis or a loss of

sensitivity by the enzyme to the usual factors restricting its activity

(Sheppard, 1964; Calvo and Calvo, 1967). In either case, the effect could

be an oversynthesis of a particular amino acid in terms of the requirements for protein synthesis and hence some quantity of that amino acid

present as the free amino acid. No mutation of this type has ever been

identified in-higher plants, but the possibility should be considered.



Serious attention should be given to the identification and utilization of

mutant genes that raise the concentration of the limiting amino acids in

both cereals and legumes that are important sources of protein for

humans and livestock. The improvement of the nutritional quality of



traditional foods has many advantages, particularly in developing nations

where it may be difficult to reach large segments of the population with

nutritional supplements.

The factors tending to enforce a relatively uniform amino acid composition for a species have been emphasized as an essential background for

those contemplating research in improving protein quality. Considering

the restrictions on change in amino acid composition, there still exist

opportunities to effect improvement in all the cereals where large quantities of alcohol-soluble proteins are synthesized. The possibility of improvement in the legumes appears less good, although a recent intriguing

report of heterogeniety in methionine content in different fractions of

soybean storage proteins may indicate that progress could also be

made here.

From theoretical considerations involving the genetic control of protein synthesis, it is probable that the most probable avenue to important

changes in amino acid composition involves changing the relative proportions of metabolically inert storage proteins that have quite different

amino acid compositions. This view has been reinforced by a study of the

effects of the o2 and f12 mutations in maize. These mutations enhance

markedly the nutritional value of maize seed proteins. Other possibilities

of effectively changing the amino acid composition of cereal grains

toward improved nutritional quality involve increasing the germ:endosperm ratio or mutations that relieve the constraints ordinarily regulating

the amount of an essential amino acid synthesized.

The supply of readily available plant protein may also be increased by

the selection of strains lacking or low in toxic substances that must be

destroyed or extracted before the seed proteins of cotton and many

species of the Cruciferae can be utilized.


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G. D. Swincer, J. M. Oades, and D. J. Greenland

Waite Agricultural Research Institute, University of Adelaide, South Australia


I. Soil Carbohydrates

11. The Significance of


111. Studies on Soil Polysaccharides .............................................................

B. Extraction of Polysaccharides from Soils

C. Purification of Soil Polysaccharides ..................................................

D. Fractionation of “Purified” Soil Polysaccharides ................................

E. Properties of “Purified” Soil Polysaccharides

F. The Origin, Synthesis, and Decomposition of

IV. Methods for the Analysis of Complex Polysaccharide Materials ..................

A. Introduction .................................................................................

B. Analytical Methods .......................................................................

C. Separation Methods ...

V. Summary and Conclusions

References .........................................................................................

















I. Soil Carbohydrates

Although much is known about the nature and function of many polysaccharides synthesized by individual organisms, there is little information relating to the polysaccharides produced in an environment such as

the soil which in a unique way brings together a great variety of biological

forms. The comparative neglect of soil polysaccharides is perhaps surprising when it is realized that soils not only support the majority of

higher plants but are the chief habitat for microorganisms. The amount of

polysaccharide material added to soils as plant residues or synthesized

in them by microorganisms must be enormous. Evidence that at least

some of the polysaccharides produced in soils are capable of improving

the stability of soil aggregates and therefore of encouraging the maintenance of an agriculturally favorable structure has provided the main

stimulus for the study of these compounds. The slowness of progress can

be attributed largely to the technological difficulties inherent in the

study of any system as complex as the soil.




The carbohydrates of soil are composed of a wide range of monosaccharides. Hexoses, pentoses, various deoxy and 0-methyl sugars,

uronic acids, and amino sugars have been identified in hydrolyzates of

numerous soils and soil extracts (Mehta et al., 196 1 ; Gupta, 1967). The

presence of such a variety of components makes precise measurement of

total soil carbohydrates very difficult, and this difficulty is aggravated by

the low stability of most of the carbohydrate monomers under conditions

so far found necessary for their release from polymeric compounds. However, the quantitative determinations that have been made indicate that

carbohydrates constitute between 5 percent and 25 percent of the soil

organic matter.

Free monosaccharides constitute less than 1 percent of the soil carbohydrates, and extracted polysaccharides have rarely accounted for more

than 20 percent (Mehta et al., 1961; Gupta, 1967). Approximately another 10 percent may consist of cellulose (Gupta and Sowden, 1964).

More recently, techniques have been developed that enable almost complete extraction of carbohydrates from soil (Swincer et al., 1968a,b). The

composition of carbohydrates removed by vigorous extraction procedures

is similar to that of materials removed by less efficient methods, and the

reason for differences in the ease of extraction of polysaccharides from

different soils would appear to be physical rather than chemical.

I I . The Significance of Soil Polysaccharides

The main stimulus for the study of soil polysaccharides has been the

repeated indications of their influence on soil physical conditions. The

polysaccharides undoubtedly also affect other soil properties such as

cation exchange capacity (due to the uronic acid units), the retention of

anions (due to amino groups, but only in acidic soils), carbon metabolism,

biological activity (e.g., by acting as an energy source for heterotrophs),

and the complexing of metals.

Interest in the relationship between the physical properties and the

polysaccharide components of soils was aroused by several reports which

indicated that microbially produced gums could bind soil particles into

stable aggregates (Winogradsky, 1929; McCalla, 1943, 1945; J . P.

Martin, 1945a, 1946; Geoghegan and Brian, 1946, 1948; Haworth et al.,

1946; Swaby, 1949). More recent work (Clapp et al., 1962; Harris et al.,

1963; J. P. Martin and Richards, 1963; J . P. Martin et al., 1965) has confirmed the earlier observations. The presence in the soil of organisms that

produce aggregate-stabilizing gums when cultivated in the laboratory (J. P.

Martin, 1945a,b; Forsyth and Webley, 1949; Forsyth, 1954; Bernier,

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